In the solar cell industry, it is known that germanium (Ge) is a desirable semiconductor material that absorbs substantial amounts of solar energy. Commercially, 3 junctions using III-V materials are deployed on a germanium substrate to emulate or match the solar spectrum. In these devices the higher energy of the solar spectrum (e.g. blue light) is absorbed by the high bandgap materials, such as GaAs, InGaP and InGaAs. There are major problems with the use of germanium wafers. Germanium wafers are expensive and constitute approximately 50% of the total cost of the device. Also, germanium wafers are heavy and very brittle so that they are generally limited in size to less than 6″ in diameter. Further, because the wafers are brittle they must be relatively thick which due to the thermal conductivity issue creates a cooling problem.
Presently, it has been found that the addition of tin (Sn) to germanium extends the absorption spectrum of a solar cell into lower energy light. Some efforts have been made in the prior art to grow GeSn incorporating a constant mole fraction of Sn on silicon substrates but the thickness of the layers is limited because of cracking and stress fractures. Because of the poor quality GeSn, efforts to grow IV and III-V materials used in solar cells has resulted in poor interfaces between materials and poor quality IV and III-V materials. As an example, a description of one such prior art method can be found in U.S. Pat. No. 7,589,003, entitled “GESN Alloys and Ordered Phases with Direct Tunable Bandgaps Grown Directly on Silicon”, issued Sep. 15, 2009.
Because of the poor quality GeSn, efforts to grow IV and III-V materials used in solar cells has resulted in poor interfaces between materials and poor quality IV and III-V materials. Generally the poor interfaces and the poor crystalline quality of the solar cells that are formed results in substantially reduced efficiency and lower solar collection.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide new and improved methods for the growth of single crystal III-V solar collection materials on single crystal GeSn on silicon substrates.
It is another object of the present invention to provide new and improved methods of growing III-V solar collection materials on GeSn with improved interfaces between materials.
It is another object of the present invention to provide new and improved solar cells with increased efficiency.
It is another object of the present invention to provide new and improved solar cells that are less costly and easier to produce.